Pulmonary endothelial function is an important determinant of lung funCtion. The evaluation of certain pulmonary endothelial functions in vivo can be accomplished using multiple indicator dilution (MID) methods. Our overall objective is to continue to evaluate metabolic functions of the pulmonary endothelium in the intact lung and to develop MID methods appropriate thereto. We will evaluate the information contained in MID data for three classes of probes of different aspects of pulmonary endothelial function. The first class includes redox dyes as probes for endothelial trans-plasma membrane electron transport. The second includes ligands for angiotensin converting enzyme, ACE, as probes for pulmonary endothelial ACE activity. The third includes rapidly diffusing lipophilic amines as probes of endothelial perfusion kinematics (pulmonary capillary transit time distribution). Certain thiazine dyes are reduced on the luminal endothelial surface during passage through the pulmonary circulation. This reduction process involves trans-membrane electron transfer from intracellular electron donors. Dye reduction is followed by uptake into the cells and sequestration in intracellular organelles. In Specific Aim 1, we will attempt to identify the kinetic processes which control thiazine dye reduction, uptake, and sequestration and the impact of changes in the lung redox state (e.g. hyperoxia, reperfusion) on these processes. We will use isolated perfused lung preparations wherein appropriate variables can be controlled and a series of dyes having differing physical-chemical properties to accomplish this aim. The kinetic analysis of pulmonary endothelial uptake of ligands for ACE, e.g. captopril, which contain a terminal proline amide bond, is complicated by the existence of rotational isomeric- forms about the proline amide bond. In Specific Aim 2, we will determine the importance of ligand isomerization using MID analysis of 18F-captopril and thereby identify the appropriate MID method for evaluating ligands of surface enzymes such as ACE using isolated perfused lung preparations. MID indicators that freely diffuse through the pulmonary capillary endothelial barrier can provide information about the amount of time the blood spends in contact with the pulmonary endothelium. In Specific Aim 3, we will exploit the MID method to measure the mean and distribution of pulmonary capillary transit times in intact dogs. To accomplish these Aims, we will express our hypotheses regarding the fate of these indicator probes on transit through the pulmonary circulation in the form of mathematical models. The ability of the models to explain (fit) the data will be a formal test of the viability of the hypotheses. In so far as the hypotheses prove to be useful, the model parameter values will provide the means for interpreting changes in the pulmonary disposition of these probes in terms of changes in the function of the endothelial cells within the intact lung.